While planning
DTV Service
circa 1996, the
FCC assumed that there
would be no interference
between a pair of
DTV signals on channels
which for analog
transmission in the UHF
band had a “taboo” relationship.
That assumption was based on input
from the FCC Advisory Committee on
Advanced Television Systems.

So far, so good. The commission was able
to use formerly taboo channels for DTV
and to my knowledge, this has not caused significant interference. This allowed planners
to use what had been a taboo channel
to relocate broadcasters during the transition from NTSC to DTV.

Let’s assume there was a station transmitting
NTSC on Channel 30. Now it is
transmitting DTV signals on Channel 30
from that site. Another station in that community
may have been allocated Channel 44 or 45 for DTV transmissions. This
station’s transmitter might be anywhere
in the community. That would be OK for DTV, but would not have been permitted
for NTSC because both Channels 44 and
45 have a taboo relationship with Channel
30. Under NTSC rules, NTSC transmitters
could not be less than 10 miles apart if
they operated on a pair of taboo channels.

With repacking now in the future for
many UHF broadcasters, the problems
of interference between DTV signals
crammed into Channels 14–29 for example,
need to be reconsidered. That is why
my colleagues and I have carried out laboratory
experiments to quantify the robustness
of actual DTV receivers to undesired
signals on taboo channels.

Fig. 1: Dmin with single interferor for 26 converters. The Noise
Limited receiver sensitivity equals–85 dBm in this figure. With
interference, Dmin increases by the mean value (–71 dBm for N+3)
for 50 percent of receivers. For 84 percent of receivers,
Dmin is 4.9 dB higher. Interference on N+3, N+9, N+14
or N+15 may also be significant.

SO WHAT WERE THESE UHF
TABOOS?
First, those taboo channel relationships
are with respect to the desired channel
N. The analog taboos were N+/–2, +/–3, +/–4, +/–5, +/–7, +/–8, and +14 and +15.
These last two so-called “image channels”
deserve some explanation. All TV receivers
employ the superheterodyne principle
invented in 1917 by Major Edwin
Armstrong, who was trying to detect the
ignition noise of enemy airplane engines.
Such ignition noise is very faint at the distances
at which detection was needed,
tens of miles.

Armstrong knew that vacuum tubes
could provide gain at low frequencies
much better than they could at higher
frequencies so he downconverted the
ignition noise (RF) to just above audio
frequencies. He called his circuit a “superheterodyne,”
and that name stuck. In the
early 1920s he designed very sensitive
radio receivers on this principle. The antenna
circuit was tuned to the desired station’s
frequency, and this was fed to the
frequency conversion device, a vacuum
tube called the “mixer.” Each superhet had
a local oscillator and that was tuned to a
frequency above the desired station by
what he called the “intermediate frequency,”
typically then 50 kHz. The local oscillator
(LO) sine wave overloaded the mixer
tube to generate second order distortion
products: F osc. + desired frequency; and F
osc. – desired frequency.

He then amplified the difference frequency, which was around 50 kHz and
detected the amplified IF signal.

However, he quickly discovered that if
there were another station 100 kHz above
the desired station in frequency, he heard
both. He realized that his superhet was
equally sensitive to signals above and below
the frequency of the LO. He called this
spurious (meaning
unwanted) response
an “image”
and that name
stuck.

Now consider
a DTV station on
Channel 30 (N)
whose center
frequency is 569
MHz. The LO to
receive this signal
must be tuned to
44 MHz plus 569
MHz to get the difference
frequency
of 44 MHz which is the intermediate
frequency (IF) of TV receivers in North
America. Alas, if there is a station on Channel
44 (N+14) centered at 569 MHz + 14*
6 MHz = 653 MHz, or a station on Channel
45 (N+15) whose center frequency is 569
MHz + 15* 6 MHz = 659 MHz, that signal
will also be heterodyned to the 6 MHz
wide IF band centered at 44 MHz. These
signals will all pass thru the IF amplifier
to the second detector. Being DTV, undesired signals appear as noise in the output
of the IF amplifier. This noise from the image
signals adds to the receiver-generated
noise and the grand total of all noise must
be at least 15.2 dB below the desired signal
level or reception fails.

Analog TV tuners typically had one
tuned circuit between the antenna and the RF amplifier, and two tuned circuits
between the RF amplifier and the mixer.
These were tuned to the desired channel
and they attenuated signals 14 or 15
channels higher than the desired channel.
The FCC established a minimum spacing
between analog TV transmitters on channel
pairs such as N and N+14/15 so that
with the RF selectivity of the tuned circuits
in analog TV sets, interference was
prevented.

For DTV planning, the FCC believed
that no rules were needed to prevent
DTV-DTV interference. That is, they believed
that DTV receivers would be designed
to reject such interference. The
FCC rules speak of a desired to undesired
signal power ratio (D/U).

Designers of tuners seized upon the
notion that the FCC
said that DTV-DTV
interference was
improbable, and
designed DTV tuners
with little or no
RF selectivity. This
is why we have run
experiments with
two and more DTV
signals to see what
happens with DTV
receivers. We tested
26 NTIA-approved
DTV converter boxes,
(there are about 30
million NTIA-approved converter boxes
in American homes providing DTV reception).

We found that these receivers can lock
to a desired signal and display pictures
down to –85 dBm. The FCC estimated
the D/U ratio for DTV-DTV interference
to be –63.00 dB, so the maximum undesired
signal power could be –85 dBm + 63
dBm = –22 dBm. So we set the U power
to –20 dBm and found that with either image channel present, the receivers were
being desensitized by about 8 dB—half
were below average and the others
above average. What it means is that
with desensitization by strong undesired
signals, people receiving a weak desired
signal now may lose reception of that
(weak) signal when after repacking, interference
is caused by strong undesired
signals, especially image signals on N+14
and/or N+15.

Fig. 1 shows our test results. The blue
squares represent the loss for 50 percent
of the units tested. The magenta squares
represent one standard deviation (SD).
One standard deviation from the mean
84 percent of the population should
have reception.

In Fig. 1, the mean (50 percent) values
for N+14 and N+15 are about 8 dB.
As one SD equals 3.6 dB, it follows that
68 percent of the people would have reception
provided their desired signal is
received 12 dB above –85 dBm, statistically
speaking.

Significant interference from a single
U signal on channels N+/–2 or +/–3 also
was found in our tests as is also shown
in Fig. 1. More on this in my next column.
Stay tuned.

Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached
via e-mail atcwr@bootit.com